OK, I'll try to explain a man's thinking or non-thinking, which can cause this tragedy.
I'm an older male with lots of experience with marriage (32 years) and never cheated. But, I have been tempted before and I think in the cases here that StacyHoll may be right.
Younger men get scared when a baby is on the way. Most of them accept the responsibility, but a few do not. So, they look around for an easy way out and now, with the internet, they can connect easily to lots of girls. The really sad fact is that he says online that he is single, which to me doesn't mean he just wants to talk.
You can't trust any guy who will do this behind his partner's back. Personally, I wouldn't see them as marriage material anymore if I were you because who knows what will happen when they have to face other problems in the future with you. The fact that you both were pregnant is incredible, but shows that this is not a man you want helping to raise your baby.
Trust is the most precious asset for a couple, so handle the situation accordingly. Stacy, I agree with your anger. You have a right to be mad and so does Schroader.